1. Field of the Invention
The present invention relates to a hydraulic driving mechanism of a piston and cylinder type and more particularly pertains to a stroke-adjusting driving mechanism in which the stroke of a reciprocating member, such as a piston is adjustable for use in machines such as, for example, hydraulic presses and press brakes.
2. Description of the Prior Art
In machine tools such as presses and planing machines for example in which a tool or a ram having a tool is reciprocated by a hydraulic cylinder or motor having a piston and piston rod, it is necessary to adjust the stroke length of the piston rod to stop the tool at a desired position. Also, it is often necessary to stop the tool at will in order to adjust the stroke length of the piston rod. In hydraulic presses for bending sheet metals, for example, it is necessary to precisely adjust the stroke length of the ram to stop an upper tool at an optimum position with regard to a cooperating lower tool depending upon the thickness of sheet metals to be bent and the angles to be formed in the sheet metals. Also, it is often desired to stop and move at will the ram in initially setting the tools and adjusting the stroke length of the ram particularly in so-called air bending operations, for example.
Heretofore, several measures have been employ to stop the piston and piston rod in hydraulic driving mechanisms for machines such as hydraulic presses. For example, an actuating means such as a micro switch is so provided that it will transmit a signal to a solenoid operated valve assembly to stop the piston and piston rod when the valve assembly is actuated. As another example, the piston is so arranged as to be forcibly stopped by a stop member with a force exceeding the hydraulic force of the hydraulic motor. Also, in another conventional technique, a valve is disposed in the hydraulic circuit for urging the piston and piston rod so that it may exhaust the hydraulic fluid to stop the piston and piston rod when the piston rod has reached a predetermined position.
However, all of the conventional measures for stopping the piston and piston rod have suffered from serious shortcomings. In the use of an actuating means such as a micro switch, a considerable time elapses until the piston is stopped after the actuation of the actuating means, and therefore the piston rod cannot be stopped at a desired position.
In case of forcibly stopping the piston rod with the use of a stop member, a large force is required, and for this reason it is impossible to stop the piston rod while maintaining a force on the piston with the hydraulic fluid. Also, in the technique of using a valve for exhausting the hydraulic fluid, the valve will experience variations in opening area depending upon the hydraulic pressure, and therefore the piston and piston rod cannot be stopped at a desired position. Furthermore, in the case of any of the conventional techniques, it is impossible to stop and move as necessary the piston and piston rod, let alone stop the same while keeping the piston pressed by the hydraulic fluid.